Checked LINQ queries and collections for inefficiencies.

- Made Array.IndexOf available via extension method.
- Made ToHashSet extension method.
- Change collections queried often via Contains into sets.
- Avoid Count() extension if Count or Length property exist.
- Made Count() > 0 checks and variations calls to Any() instead.
- Don't call ToList/ToArray if there is no benefit to materializing the sequence.
- If the sequence does benefit from materialization, follow this general pattern:
  - Collection queried often via Contains use ToHashSet to speed up lookups.
  - Short lived variables use ToList. This is because ToArray requires an extra copy to output the final size.
  - Collections persisted into fields or for a long time use ToArray to minimize memory overhead.

OpenRA.Mods.Common/Widgets/ProductionPaletteWidget.cs

@@ -281,7 +281,7 @@ namespace OpenRA.Mods.Common.Widgets
 					Hotkey = ks.GetProductionHotkey(DisplayedIconCount),
 					Sprite = icon.Image,
 					Pos = new float2(rect.Location),
-					Queued = CurrentQueue.AllQueued().Where(a => a.Item == item.Name).ToList(),
+					Queued = CurrentQueue.AllQueued().Where(a => a.Item == item.Name).ToList()
 				};
 
 				icons.Add(rect, pi);